April 2001doc.: IEEE 802.11-01/215
IEEE P802.11
Wireless LANs
Tiered Transmitter Power Control (TTPC)
Proposal for 802.11h WLAN
Date:April 10, 2001
Author:Peter Larsson, Ericsson Radio Systems AB
e-Mail:
Abstract
This submission contains a proposal for a transmit power control (TPC) scheme to be included into the IEEE 802.11 standard. In designing the methods for TPC, it has been instrumental to ensure, not just compliance of European Radio communications Committee (ERC) regulatory requirements in the 5 GHz band, but also to provide precise and fast TPC mechanisms enabling efficient network operation and savings of resources. While the proposal defines frame exchange sequences, formats, and rough guidelines for a TPC policy, it also make certain that the implementers retain their freedom to design and incorporate their own algorithms of taste.
The proposed method calls for some minor amendments to the 802.11 MAC as well as 802.11a PHY specifications.
1Introduction
European Regulatory requirements for the “5 GHz band” defined by ERC, limit the mean EIRP to 200 mW and 1 W, in the 5150-5350 MHz (indoor) and the 5470-5725 MHz (indoor and outdoor) band respectively. Further, DFS shall be applied over both bands in conjunction with TPC, the latter operating both in down- and uplink. IEEE 802.11 devices operating in the ERC area must therefore comply with stated conditions. As the IEEE 802.11 standard currently does not incorporate the required TPC mechanisms, it is an objective of this submission to bring forward necessary methods with respect to TPC, such that ERC directives can be fulfilled.
The TPC proposal targets both infrastructure-based 802.11 WLANs with an AP, or Infrastructure BSS, as well as ad hoc-oriented 802.11 networks, or independent BSS (IBSS). As today, DCF has been the preferred mode of operation as well as being the fundamental channel access mode of 802.11. It is therefore felt that the development of an efficient TPC scheme should be based on the foundation of DCF. Moreover, although [2] presented a TPC scheme with the ability of individual STA control within the PCF operation, only a fixed TPC approach was suggested for DCF. For some margin of ERC compliance and the ability of occasionally using full transmit power, we do however believe that DCF also needs to be addressed with respect to TPC. Against this background, DCF establishes the foundation for our TPC proposal. However, the proposed mechanisms could be extended to support the (E)PCF or the HCF mode.
It is our belief that the TPC scheme should, besides meeting the ERC requirements, have as goals to prolong battery lifetime, improve throughput and delay characteristics, implicitly enhance experienced QoS and reduce the need of overlapping BSS handling mechanisms, [1]. In achieving those objectives, it is judged instrumental to provide TPC mechanism of both precision and speed. With this in mind, the standard specification should define the frame formats, fundamental TPC mechanisms and coarse policies, but still enable implementers to decide on the specifics of algorithms.
In this submission, we propose TPC for IEEE 802.11 with some modification in the current 802.11 MAC and 802.11a PHY specification.
As it is the basic operational principles of the TPC scheme that is primarily targeted here, the expertise of the group and of the editor are invited to help finding a way forward in defining the protocol details.
2Tiered TPC
In reducing generated interference and minimizing power consumption, it is vital to apply the most aggressive and precise TPC scheme to the bulk traffic of the network, most likely consisting of Data (and ACK) frames. Next to Data frames, the RTS and CTS frames may, depending on the dot11RTSThreshold value, be relatively prevalent and hence considered as important contributors to undesired interference and power consumption. As RTS and CTS frames in general are shorter than Data frames, their supplement to the overall average interference picture will accordingly also be lower. Frames occurring merely occasionally, such as Beacons, have even less impact on the average interference situation. In addition, of diminishing the radiated average interference level, the issue of minimizing peak interference and associated variations is also of interest. Different traffic conditions may alter assumptions above, but the given statements are believed to be true in most, if not all relevant scenarios.
Those issues together with the objectives set forth earlier motivate a so called TPC policy to be defined, giving very rough guidelines on TPC algorithmic goals. Outgoing from the TPC policies, a TPC mechanism is defined.
It is worth emphasizing that although the proposed TPC mechanisms is designed with the TPC policy as a foundation, nothing will prevent implementers to adopt less precise or aggressive algorithms at their own liking. However, the TPC policies together with proposed TPC mechanisms attempt to provide a cornerstone enabling very efficient operation of 802.11 systems.
The TPC policy follows a tiered approach defining three levels. Frames with different topological destination objectives are divided among those three Tier-classes.
As Tier 1 frames are sent with high transmit power, this class also adopts a policy of being constrained in time. The reason being minimizing random interference peaks within and towards neighboring (I)BSS. This is achieved by confining Tier 1 traffic around Beacon transmit occasions, i.e. in essence at TBTT.
1 / Outside and within (I)BSS. / Beacon,
Probe Request [1],
Probe response[2],[3] / PTX Highest permitted domain transmit power
Definition range > Start of the Beacon
Definition range < Immediate vicinity after the Beacon.
2 / Within (I)BSS. / RTS, CTS, 3 / PTX Tier 1
PTX > max required PTX to any STA in (I)BSS
Outside Tier 1 definition range
3 / Directed to single STA / DATA, ACK, 3 / PTX Tier 2
PTX > Required PTX to desired user
Outside Tier 1 definition range
Table 1 TPC Tiers
The mechanism to acquire information for tiered TPC is simple. The procedure for each Tier TPC method is discussed below.
2.1Instantaneous Closed Loop TPC
This section defines the TPC method for frame subtypes belonging to Tier 3.
Each node receiving an RTS message assesses preferably the instantaneous carrier to interference ratio, CIR. Subsequently, a desired reduction or increase of transmit power relative the transmit power used for the RTS frame, is determined. A relative transmit power adjustment request, PTX_Request is then conveyed in the CTS frame back to the originating STA. The originating STA adjust the transmit power level accordingly for the subsequent DATA frame transmission. The same procedure is repeated for the ACK, i.e. the originating STA conveys a corresponding transmit power adjustment request targeted for the ACK. Note that, both RTS and CTS are sent with TPC Tier 2 related mechanisms as described in 2.2.
The details of the algorithm in selecting the transmit power is implementation specific, but the mechanism as such inherently enables an very precise adjustment with respect to instantaneous experienced CIR at the receiver. An example of the given flexibility is that at choice, the implementation specific algorithm may respond with a PTX_Request, forcing the sender of Data to use maximum permitted domain transmit power. While this is indeed possible, it generally results in a poor-performing system, as spatial reuse is reduced and power consumption increased.
The modification of the CTS and Data frames may be accomplished as part of the TGe process or independently in TGh. See 2.4. for details regarding the frame structure.
Note that, in sending short Data or Management frames that do not exploit the RTS-CTS scheme, information derived from the open loop group TPC as described in 2.2 is utilized.
2.2Open Loop Group TPC
The open loop group TPC addresses the Tier 2 TPC policy. The aim is to determine a transmit power setting such that all STAs within an (I)BSS will have a sufficient CIR enabling each one to receive Tier 2 frames.
This mechanism is also useful for TPC setting for broadcast and multicast traffic within the (I)BSS, but it is primarily aimed towards RTS and CTS frames.
The need of this mechanism requires some clarifications. While the RTS-CTS frame exchange efficiently prevents hidden stations to access the channel, further enhanced by the virtual carrier sense, the RTS-CTS frames themselves need to be protected with classical physical carrier sense. As a result, it is vital to ensure that all STAs within the same (I)BSS transmit with sufficient power so as to reach each other. However, from the viewpoint of interference and power consumption, it is preferred to send with the least possible transmit power. The Group oriented TPC proposed here intend to strike a balance between those two somewhat conflicting goals.
IBSS
The procedure for IBSS Group TPC is based on conveying transmit power level information, PTX, as an information element, IE, in the regular IBSS Beacon. Hence, PTX merely represents the transmit power employed for the frame in which the IE itself is transferred within. The intent of using the Beacon is because it complies well with both power save mode operation as well as the Tier 1 objectives. In addition to the transmit power level information, a minimum required receive power level, PRX_min is sent in the same IE.
Each STA receiving a Beacon with the IE determines path gain and subsequently required transmit power. Each STA also assess that the Beacon originates from an STA within the same IBSS. Over the time, as the IBSS Beacon transmit time is somewhat randomized, Beacons from all STAs within the same IBSS and within range will be received. Based on the collated information, the maximum required transmits power is selected among the STAs. Old transmit power updates lose in validity over time, as corresponding new updates are not overheard.
Note that the scheme allows the implementers to adjust PRX_min to their liking.
BSS [4]
The procedure for BSS group TPC is somewhat similar to the procedure for IBSS, but the channel probing sequence is directed by the AP.
A transmit power information request directed towards a selected STA is issued by the AP. This request is sent via an IE carried in a Probe request just immediately after the Beacon. Subsequently, a Probe response is sent from the addressed STA with another IE indicating the used transmit power information PTX and a minimum required receive power level, PRX_min. The Probe request and Probe response employs the Tier 1 TPC policy.
Each STA receiving the Probe response with the IE, determines path gain and subsequently required transmit power. Each STA also assesses that the frame originates from an STA within the same BSS. Over the time, frames with the desired IE from all STAs within the same BSS and within range are received. The maximum required transmit power is then selected among the STAs with respect taken to changing channel gain over time.
The polling sequence of STAs belonging to a BSS is an implementation specific issue and not defined in the standard. As for the IBSS, the scheme allows the implementers to set PRX_min in preferred manner.
Note, that by regulating PRX_min, STAs will try to adaptively compensate desired receive power in the presence of an adjacent interfering BSSs. Hence if maximum domain transmit power is the optimum, the system shall select transmit power parameters accordingly. In contrast, other situations will conserve the resources instead.
2.3Broadcast TPC
This section defines the TPC method for frame subtypes belonging to Tier 1. The domain specific transmit power approach introduced in [2] is adopted here. See [2] for specifics pertaining to domain specific settings.
Each STA within an (I)BSS uses the allowed domain transmit power when sending a frame (subtype) containing any of the IE as defined in 2.4. If the transmit power capabilities is less than the domain transmit power level, the former must be used.
2.4Frame and IE formats
Two new IEs are defined. The first one is used to request a frame with transmit power information. The second one is used to indicate which transmit power is used in the frame it is sent within.
Octets: 1 / Octets: 1 / Octets: 0Element ID / Length / Transmit Power Information Request
Figure 1 Transmit Power Information Request Element
Octets: 1 / Octets: 1 / Octets: 1 / Octets: 1Element ID / Length / Transmit Power Information, PTX / Minimum required receive power, PRX_min
Figure 2 Transmit Power Information Element
Management frame subtype Beacon is modified to include two new IEs. The transmit power Information element is only used in IBSS operation.
Order / Information / Comments11 / Domain Information / See [2]
12 / Transmit Power Information Element / Only in IBSS operation
Figure 3 Beacon modifications
Management frame subtype Probe request and Probe response is modified to include one new IE each. This is merely used in BSS operation.
Order / Information / Comments3 / Transmit Power Information Request Element / Only in BSS operation
Figure 4 Probe request modifications
Order / Information / Comments10 / Transmit Power Information Element / Only in BSS operation
Figure 5 Probe response modifications
The inclusion of a field for closed loop Transmit Power Control in CTS and Data/Management frames are shown in Figure 6.
Figure 6 Frame structure
B0-B1 / B2-B7Reserved / CL-TPC info: 1 dB steps
Figure 7 PTX_Request
3Conclusion
A Tiered Transmit Power Control scheme, able to operate in both BSS and IBBS network, on basis of the fundamental DCF channel access scheme has been outlined. For Tier 3 traffic, i.e. directed Data and Management frames etc generating the bulk interference and being the main source of power consumption, precise and instantaneous TPC based on the closed loop paradigm is adopted. For Tier 2 traffic, i.e. CTS and RTS frames etc, an open loop strategy ensures that STAs belonging to the same (I)BSS see the channel as one resource while retaining Inter (I)BSS interference on lowest possible level. Tier 1 traffic, BEACON frames etc., enable path gain and estimation of required transmit power for tier 2 traffic. In addition, Tier 1 traffic confine itself in time so as to enable predictable and short lasting interference peaks.
It is argued that an adaptive tiered and fast TPC as presented, has potential to significantly reduce interference, prolong battery lifetime and improve system performance. In addition, the freedom of implementers to decide on algorithm specifics can be preserved.
4References
[1]Peter Larsson, Ericsson Research. “Some additional thoughts on TPC”, doc.: IEEE 802.11-01/160.
[2]S. Choi et al. Philips Research, Nokia Research Center, and ComNets Aachen University, “Transmitter Power Control (TPC) and Dynamic Frequency Selection (DFS) Joint Proposal for 802.11h WLAN”, doc.: IEEE 802.11-01/169.
Submissionpage 1Peter Larsson, Ericsson Radio Systems AB
[1] When used in BSS with IE defined in Figure 1
[2] When used in BSS with IE defined in Figure 2
[3]Details of other frame subtypes is TBD.
[4] Observe that sending a Probe request and Probe response just after the Beacon is not very efficient. One possible alternative is to make use of the Beacon in which the Probe request IE (transmit power request information) is then conveyed. A Probe response or another more suitable frame then conveys the transmit power information IE.